It has become quite popular to form images on plain or treated papers by the imagewise deposition of inks. This deposition can take place by means of contact or impact printing, as in a printing press or typewriter like arrangement or by a variety of more modern non-impact printing systems. One of these non-impact printing systems is known as ink jet printing.
In ink jet printing, tiny droplets of ink are projected directly onto a receptor surface for printing without physical contact between the printing device and the receptor. The placement of each drop on the printing substrate is controlled electronically. Printing is accomplished by moving the printhead across the paper, or by moving the paper across the printhead.
Different types of ink jet printing are known. Two major forms of ink jet printing are "drop-on-demand" printing and "continuous jet" printing. Continuous jet printing is characterized by pressure-projecting inks through a nozzle to generate drops of ink directed in a continuous stream towards the ink receiving element, while passing through an imagewise modulated ink deflection system, thereby allowing ink droplets of the stream to deposit imagewise on the recording element. Drop-on-demand or impulse ink jet differs from continuous ink jet in that the ink supply is maintained at or near atmospheric pressure. An ink drop is ejected from a nozzle only on demand when controlled excitation coming from pressure generated by a piezoelectric element or from pressure generated by local electrothermal evaporation of liquid (thermal bubble jet) is applied to an ink filled channel ending in a nozzle. Acoustic, microfluidic, and electrostatic driven drop-on-demand techniques are also known. These technologies are described in detail by J. L. Johnson, Principles of Non-Impact Printing, Palatino Press, Irvine, Calif. (1986), and in Neblette's Imaging Processes and Materials, Eight Edition, J. Sturges Ed. Van Nostrand, New York, (1989).
When several ink streams are independently employed to imagewise deliver colored inks to a surface, color images can be obtained The inks employed for this purpose typically fall into one of two categories, pigmented inks and soluble inks. The pigmented inks have the advantage of providing stable color images but are lacking in that the pigment particles rest at the surface of the receiving element and are especially prone to mechanically induced smear and rub-off. Additionally, heads delivering the pigmented inks are prone to clogging. The soluble inks solve the rub-off and clogging problems but suffer in that they are prone to both thermal and light fading and to image smearing in humid environs or when the receiving element is hand handled or otherwise wetted.
In related art, Oelbrandt et al in U.S. Pat. No. 5,621,448 describes the imagewise application of a reducing agent solution to a receiving element having a reducible silver salt to imagewise form a metallic silver image. The possibility of intensifying this black image by the presence of color coupler dyes is mentioned. Sambucetti and Seitz, in IBM Technical Disclosure Bulletin vol. 20, pages 5423-4 (1978), describe the formation of images by imagewise applying a jet or mist of a reactive species to a paper impregnated with a reactant to again form metallic images. Leenders et al in U.S. Pat. No. 5,621,449 describes imagewise applying a reducing agent to a receiver element comprising a reducible silver salt to form a metallic silver image. The possibility of intensifying this black image by the presence of color coupler dyes is mentioned. The methods described by these workers are directed at providing black images which, in some cases, may be intensified by the presence of color couplers. These methods all suffer in that the receiving element or the imagewise mist must contain between them sufficient developing agent and metal salts to form a dense image, thus requiring that large quantities of solution be employed to deliver the components. The element dries slowly and forms only a black-and-white image at best. Pimbley, in IBM Technical Disclosure Bulletin vol. 23, pages 1387 (1980), discloses that leuco dyes or vat dyes can be applied to a paper coated or impregnated with an oxidizing agent. This method suffers in that the leuco or vat dyes are unstable and, thus, leads to a material having poor shelf life. Sufficient details to practice this disclosure are not revealed.